Customer requests an R-407C drop-in retrofit of an R-22 packaged air conditioner
An old 5 TR packaged air conditioner serving a small office is charged with 3.2 kg of R-22 and uses mineral oil with a thermostatic expansion valve. The unit has a small leak at an evaporator flare and has needed top-ups twice this year. The customer has heard that R-407C is a ‘drop-in’ replacement and asks you to recover the R-22, replace the filter drier, evacuate, and charge R-407C without changing the oil. The compressor is a hermetic reciprocating type, 415 V three-phase, and still runs; cooling capacity is down and the sight glass shows bubbles. The office cannot close, and the customer wants the job done in one afternoon. R-22 supply is becoming expensive and the customer is also concerned about environmental rules. The nameplate states R-22 only; no retrofit instructions are available on site.
1. Evaluate the proposed R-407C drop-in. What is technically wrong with the plan, and what would you check before quoting?
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R-407C is a zeotropic blend with a significant temperature glide, not a true drop-in for R-22. It must be charged as a liquid, and the evaporating and condensing temperatures must be interpreted with glide in mind. The existing mineral oil is the bigger problem: mineral oil has poor miscibility with R-407C, so oil return to the compressor may be unreliable. A polyolester oil or the manufacturer's approved lubricant is normally required, together with a new filter drier and often a TXV adjustment.
Before quoting, check the compressor manufacturer's retrofit instructions, the oil type and charge, elastomer compatibility, pressure ratings, TXV bulb charge and adjustment range, and whether the existing line sizes suit R-407C. The leak at the evaporator flare must be repaired first; recovering and recharging without repair would waste refrigerant and leave the same fault. The bubbles in the sight glass may indicate undercharge from the leak, not simply a refrigerant problem. Also check that R-407C is actually available and permitted for this application; do not promise a one-afternoon job before the retrofit data are confirmed.
2. Describe a correct retrofit procedure and the environmental compliance points you would explain to the customer.
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Repair the evaporator flare leak first, then recover the R-22 using certified recovery equipment; never vent it to atmosphere. Measure and record the oil removed. Drain the mineral oil, flush if the manufacturer requires it, and install the approved POE lubricant and a new filter drier. Pressure-test with dry nitrogen, evacuate with a micron gauge, and hold the vacuum. Charge R-407C by weight as a liquid, then adjust the TXV to achieve the manufacturer's superheat and check subcooling, current and supply-air temperature difference.
Explain that R-22 is an HCFC with ozone-depletion potential and is being phased out, so recovery and leak repair are mandatory and good practice. R-407C does not deplete ozone in the same way but still has global-warming potential, so leaks must be minimised. Label the unit clearly with the new refrigerant and oil, record the charge, and give the customer a written note that the system is no longer R-22. If the retrofit data or approved oil are unavailable, offer to repair the leak and recharge with R-22, or quote a replacement unit, rather than proceeding blindly.
3. Which commissioning readings would show that the retrofit has been successful?
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At full load, check suction and discharge pressures, evaporating and condensing temperatures with glide allowance, superheat at the evaporator outlet, subcooling at the condenser outlet, compressor current against the nameplate, and the supply-air temperature difference. Superheat should be within the TXV setting, typically around 5-8 K at full load unless the manufacturer specifies otherwise, and subcooling should be stable with no bubbles in the sight glass. The compressor current should remain below the rated value and the unit should not short-cycle.
Also leak-test every joint, confirm that oil return is occurring, check the filter drier for a temperature drop, and record the actual refrigerant charge and oil type. Compare the supply-air temperature difference with the original R-22 performance and explain any capacity change to the customer. A successful retrofit is not just 'it cools'; it is stable superheat, correct current, no leaks, and documented refrigerant identity.